1. Field of the Invention
The present invention relates to a material for negative electrode of a lithium ion secondary battery and a process for manufacturing the same, and a negative electrode using the same and a lithium ion secondary battery using the negative electrode.
2. Description of the Related Art
In recent widespread use of mobile phones, personal digital assistants, video cameras, notebook computers, and others, there is an urgent need for improvement in performance of the batteries used in these electronic instruments. Various batteries such as lithium secondary battery, nickel hydride battery, nickel cadmium battery, and the like have been used in these electronic instruments, but among them, a lithium secondary battery is attracting attentions from the viewpoints of enablement of miniaturization and supply of high-voltage. The lithium secondary battery is a group of secondary batteries in which lithium is involved in the electromotive reaction, and more specifically classified into a lithium ion secondary battery that uses a carbon electrode as the negative electrode and a lithium cobalt complex oxide or the like as the positive electrode, and metal lithium secondary battery that uses a lithium alloy as the negative electrode. The lithium ion secondary battery employing a carbon electrode as the negative electrode is currently available in the market, but the lithium ion secondary battery with even higher performance is required.
Developments aimed at improving the performance of lithium ion secondary batteries have been intensively conducted from various viewpoints with respect to each component of the battery, such as an electrode, a separator, an electrolyte, and the like. Examples of such improvements with respect to carbon electrode constituting the negative electrode, are disclosed in Japanese Patent No.2983003 and Japanese unexamined patent publication No.2000-294243.
Carbon electrodes, which are used as the negative electrode in lithium ion secondary batteries, are generally produced by coating and drying a paste containing a graphite particle onto a current collector such as copper sheet or the like, and pressing the coated material. In particular, if a graphite in flake form (hereinafter, simply referred to as “flake graphite”) is used as the graphite constituting the negative electrode, the flake graphite particle orients itself along the surface of the current collector by pressing, resulting in lower permeability of the electrolyte. Lower permeability of electrolyte into the electrode causes a problem of decrease in the fast charging/discharging characteristics of the battery.
Other requirements in property for the lithium ion secondary battery that employs a carbon electrode as the negative electrode are, for example, initial efficiency and cycle characteristic. When a lithium ion secondary battery is charged for the first time, a part of the electric charges are consumed for forming a passive layer on the surface of the negative electrode (carbon electrode) and thus not all of the electric charges the battery received by charging are discharged. Therefore, the first discharge capacity will be smaller than the first charge capacity. The ratio of the first discharge capacity to the first charge capacity is defined as an initial efficiency. In addition, if a lithium ion secondary battery is charged and discharged repeatedly, the discharge capacity gradually becomes smaller than the discharge capacity in the first cycle. The decline in discharge capacity relative to the discharge capacity in the first cycle is called a cycle characteristic. The initial efficiency and the cycle characteristic are the important requirements in property for lithium ion secondary batteries and it is required to improve them.